This invention relates to a method for improved passive background subtraction using spatially resolved detection systems.
Active remote sensing systems transmit a photoexcitation light pulse such as a laser pulse through the atmosphere, collect backscattered and fluorescent emission generated by interaction of the photoexcitation light with a target, and spectrally and temporally resolve the collected light. Computer algorithms are used to process the collected spectral data to discriminate between species of interest, interfering species, and passive background spectra. Often the spectral signal from a species of interest is relatively weak. Proper detection then requires accurate subtraction of passive background light contributions to the measured spectra. Passive background light is light from sources such as solar radiation and artificial lighting where the light is not generated as a consequence of interaction with the photoexcitation light. Passive background light can include any light that is not generated by interaction of the photoexcitation light with a target.
In many remote sensing systems, a spectrum of the passive background is collected between laser shots. This background is subtracted from the “active” spectrum corresponding to the laser-excited emission before the analysis algorithms are employed to determine whether or not a species of interest is present. One significant problem with interleaving the passive background data acquisition with the acquisition of spectra from the desired target is that the intensity of passive lighting sources often changes even on the short timescale typically found between laser pulses. One example is artificial lighting (for example, from Hg-vapor or Na-vapor lamps) which exhibits a 60 Hz half-wave rectified intensity variation that is frequently not synchronized with the laser pulses from a LIDAR system. This leads to increased uncertainty in the magnitude of the actual signal that may be originating from the target of interest. This present invention addresses the problem of passive background correction.